In a conventional process of producing dimethyl ether, methanol is mainly used as a starting material, and the methanol is dehydrated. Recently, however, a process of synthesizing dimethyl ether directly from a raw material gas containing carbon monoxide and hydrogen was developed. According to the latter process, dimethyl ether can be produced by a progression of the chemical reaction formulae (1) and (2) as shown below in the presence of, for example, a copper-based catalyst for synthesizing methanol and a catalyst for dehydrating methanol (a catalyst for converting methanol) such as alumina. More particularly, methanol is produced from carbon monoxide and hydrogen by the catalyst for synthesizing methanol, and the resulting methanol then is dehydrated and condensed by the catalyst for dehydrating methanol to produce dimethyl ether and water. Further, as shown in the chemical reaction formula (3), the resulting water is reacted with carbon monoxide to produce carbon dioxide and hydrogen.CO+2H2→CH3OH  (1)2CH3OH→CH3OCH3+H2O  (2)H2O+CO→CO2+H2  (3)
The above synthesizing reaction is an intense exothermic reaction, and there is a problem of deactivation of the catalysts used at a high temperature. Therefore, investigations of the synthesis of dimethyl ether have drawn attention to the use of a slurry-bed reaction process having advantages that a large quantity of heat can be effectively eliminated and a temperature can be easily controlled, rather than a fixed-bed reaction process having a disadvantage that an elimination of heat is relatively difficult.
In the slurry-bed reaction process, a catalyst slurry prepared by suspending catalysts in a suitable medium oil is used. The medium oil is required to have properties such as stability under the reaction condition, and an inactivity to the reaction. Further, the medium oil is desired to have other properties such that the oil is present as a liquid at a normal temperature, and thus can easily be handled, and the oil is difficult to be solidified and cause a choking in a process.
A process of producing dimethyl ether by a slurry-bed reactor is disclosed in Japanese Examined Patent Publication (Kokoku) No. 07-057739 filed by Air Products and Chemicals, Inc. (U.S.A.). The Publication discloses a paraffinic hydrocarbon or a mixture of hydrocarbons as the medium oil for forming the catalyst slurry in a reactor of the above producing process, and a purified natural mineral oil called Witco 70 is used in Example.
Further, Air Products and Chemicals, Inc. also reported, in U.S. DOE Report (DOE/PC/89865-T6), the slurry-bed synthesis of dimethyl ether, and a purified natural mineral oil called Drakeol 10 is used therein.
U.S. Pat. No. 5,459,166 by Sunggyu Lee, et al. also discloses a process of producing a gasoline component via dimethyl ether from starting materials of hydrogen and carbon monoxide by a slurry-bed reactor. In the process, a medium oil stemmed from a natural mineral oil, called Witco 40, Witco 70, or Freezene 100 is used as a slurry medium oil.
Sunggyu Lee, et al., also reported in an article [A Single-Stage, Liquid-Phase Dimethyl Ether Synthesis Process from Syngas I. Dual Catalytic Activity and Process Feasibility, Fuel Science and Technology Int'l., 9(6), 653–679 (1991)] or other articles, a dimethyl ether synthesis by a slurry-bed wherein Witco 40 or Witco 70 is used as the medium oil.
The inventors of the present invention carried out a ring analysis of the medium oils called Witco 40, Witco 70, Freezene 100, and Drakeol 10 according to an n-d-M method (ASTM D3238) and found that a % CP value (a percentage of the number of paraffinic carbon atoms with respect to a total number of carbon atoms) of each of the medium oils was less than 70.
Further, an analysis of the molecular structures of Witco 40, Witco 70, Freezene 100, and Drakeol 10 according to NMR or the like revealed that the number of branched carbon atoms, i.e., the number of carbon atoms having 3 or more carbon—carbon bonds, is 20% or more with respect to a total number of carbon atoms, in each of the above oils.
The conventional known medium oil prepared by purifying the natural mineral oil as above has a problem that an efficiency of a reaction used to produce dimethyl ether is lowered with time. For example, U.S. DOE Report (DOE/PC/89865-T6) by Air Products and Chemicals, Inc. reported that, in a slurry-bed synthesis of dimethyl ether using Drakeol 10, an amount of dimethyl ether produced was remarkably lowered with time, and the amount of dimethyl ether produced under the same conditions was lowered to about half after about 500 hours.
It was also proved that, in a slurry-bed synthesis of dimethyl ether using a conventional known medium oil such as Witco 70 or Freezene 100, the amount of dimethyl ether produced was remarkably lowered with time, as when using Drakeol 10.
Further, when the natural mineral oil as above is pyrolytically decomposed at a high temperature, a carbon residue is inevitably produced. This means that when the natural mineral oil is used as the medium oil, the catalyst may possibly be deactivated by a coking of the medium oil.
In general, from a handling standpoint, the medium oil must maintain a flowability at a suitable temperature.
The inventors of the present invention engaged in intensive research of a medium oil capable of effectively continuing a production of dimethyl ether for a long period of time in a synthesis of an oxygen-containing organic compound including dimethyl ether by a slurry-bed reactor, and found that, among the properties of the medium oil, a % CP value and an average molecular weight of the medium oil strongly influence the properties of the medium oil.
Accordingly, the object of the present invention is to remedy the defects of prior art, and to provide a medium oil which can be used in a slurry-bed reaction process, and maintain a high production of an oxygen-containing organic compound including dimethyl ether for a long period of time.